Automatic control apparatus for mass spectrometers



April M, 195

e. P. WILSQN 2,2,4

AUTOMATIC CONTROL APPARATUS FOR MASS SPECTROMETERS Filed July 12, 1956 AMPL/F/ER RECORDER J INVENTOR. GARDNEE P. WILSON ATTORNEYS United States Patent AUTOMATIC CONTROL APPARATUS FOR MASS SPECTROMETERS Gardner P. Wilson, Pasadena, Calif., assignor to (Jon-- solidated Electrodynamics Corporation Application July 12, 1956, Serial No. 597,374

7 Claims. (Cl. 250-419) This invention relates to mass spectrometers and, more particularly, is concerned with the automatic programming of the spectrum scanning and recording of the output.

Massspectrometers are well known in the art by means of which ions produced from a gas may be separated and identified according to the ratio of their mass to their charge, hereinafter referred to as the specific mass numher. In a typical mass spectrometer a sample, for example a gas mixture, is ionized in an evacuated chamber by electron bombardment. The resulting ions are propelled by electrical potentials into an analyzer. The ions entering the analyzer are sorted under the influence of a magnetic field in accordance with their specific mass into a seriesof divergent, homogeneous ion beams. The sorted ions of the various ion beams are collected and discharged, the quantity of each kind of the ions present being measured by the amount of current that they discharge upon collection;

The mass spectrometer may be adjusted to selectively measure the quantity of ions of difierent specific mass by varying either the magnetic field or the accelerating potentials which are employed to propel the ions into the analyzer.

The record of the separately collected currents is a mass spectrogram which may take any one of several forms. Generally, arecord is made of the variation in amplitude of the collector current resulting from ions present of different specific masses. Each peak on the resulting curve must be correlated with the specific mass of the ions producing the peak.

To make the mass spectrometer automatic, it will be seen that means must be provided for introducing ions into the analyzer of predetermined controllable specific mass, recording the resulting output of the analyzer, and identifying on the recording the specific mass of the ions associated with each recorded peak of the output.

Generally, because the amplitude of the output current is adirect function of the partial pressure of the particular gas present which produces the ions being analyzed, there is considerable range in amplitude. Since the particular substance which it is desired to identify in the sample may represent a very small fraction of the total sample, it is necessary to measure the small peaks and the large peaks to the same degree ofaccuracy. Since no recording system is capable of accurately reproducing peaks over such a wide range of amplitudes, which may beof the order of 100,000 to 1, it is necessary to attenuate the signal to the recorder by fixed predetermined amounts to bring all the peaks within the dynamicrange of the recording system without losing the very small peaks.

Therefore, to make the mass spectrometer completely automatic, means must be provided for introducing variablefixed amounts of attenuation between the output of the analyzer and the recorder. Each sample generally includes a main or principal constituent which produces large peaks at known mass-to-charge ratio numbers. Thus, for each sample in which the principal constituents are known, attenuation of the main peaks maybe carried 2 out according to a predetermined plan or program, d-ifiering for each sample mixture.

Also, it is desirable in the automatic mass spectrometer device to skip certain mass-to-charge' ratio numbers. For example, no numbers occur between the specific mass number 2 and the specific mass number 12.

The present invention provides an improved automatic mass spectrometer which automatically scans through the specific mass number spectrum, recording the resulting peaks for each specific mass number. The improved automatic spectrometer further provides for identification of each peak recorded as to its related specific mass number, introduces a predetermined fixed amount of attenuation for each peak according to a prearranged programming, and provides for the skipping or speeding up of the scan as to certain preselected specific mass numbers.

In brief, the present invention contemplates automatic mass spectrometer apparatus, the mass spectrometer having an accelerator and a collector. The apparatus cornprises potentiometer means for continuously varying the accelerator potential on the accelerator and means for driving the potentiometer to continuously vary the specific mass number of the ions reaching the collector. A programmed switching circuit including a readily replaceable punched disc is driven in synchronism with the potentiometer means. Means including a variable step attenuator couples the output of the collector to an oscillograph recorder, the variable step attenuator being selectively varied in response to the pattern on the punched disc. The means for driving the potentiometer means is selectively varied in speed in response to the punched disc in the programming circuit whereby desired peaks can be passed over more rapidly in the scanning operation of the mass spectrometer.

For a better understanding of the invention reference should be had to the accompanying drawing, wherein:

Fig. 1 is a schematic diagram of the mass spectrometer and its associated automatic control circuitry; and

Fig. 2 is a fragmentary view of the programming disc.

Referring to Fig. 1, the apparatus for automatically controlling the operation of the mass spectrometer is shown in combination with one type of mass spectrometer. It will be apparent that the apparatus of the present invention is not limited, however, to use with the particular mass spectrometer illustrated, but may be employed in any type of mass spectrometer in which the mass spectrum can be scanned.

In the drawing the numeral 10 indicates generally an ion source which communicates'with a conductive'analy'zer tube 12 which is enclosed within an envelope 1 43 through the wall of which a gas inlet tube 16 projects. A- high degree of vacuum is maintained within the envelope by means of a vacuum pump (not shown) connected to the envelope.

The outlet end of the analyzer tube 12 is provided with a slot through which sorted ions are projected. An electrode 18 serves to collect the ions, producing a current which is amplified by an amplifier 20. The amplifier 20 is Class-A and has a wide dynamic or linear range, of the orderoi 100,000 to 1, between the level of input at which the amplifier overloads and the noise level, by which an output voltage is produced which is proportional to the rate at which ions impinge on the electrode 18 The gas inlet tube 16 is composed of insulating material and communicates with an inlet electrode 2.2 in the form of a metallic tube which is partially closed at one end by a closure plate having a transfer slot 24 to accommodate an electron beam. The electron beam may be produced and projected through the slot 24 in any suitable manner, such as from a thermionic source (not shown). The molecules of the gas sample which are admitted into the ionization chamber through the inlet 16 are bombarded by the electron beam passing through the slot 24 and are thus converted into ions. These ions are propelled as an unsorted beam into the analyzer tube 12 by electrical potentials established between electrodes 26 and 28 in the ion source 10. The electrodes 26' and 28 have slots which are aligned with the slot 24 and the tube 22 so that the ions are propelled from the outlet 24 of the tube 22 into the analyzer tube 12 under the influence of the electrical potentials which are established between the tube 22 and the electrode 26, and between the electrode 26 and the electrode 28.

The ions enter the analyzer tube 12 as a homogeneous,

tube is immersed. The field is produced by suitable magnet means not shown.

A source of potential (not shown) is connected across a voltage divider 42. A potentiometer 44 and a potentiometer 46, connected in series, are selectively connected to two taps on the divider 42 by a relay switch 48 to provide the accelerating voltages for the electrodes of the ion source 10. The tube 22 is connected to the variable tap of the potentiometer 44, and the electrode 26 is connected to the variable tap of the potentiometer 46.

The wiper arms of the potentiometers 44 and 46 are driven simultaneously by a motor drive 62 which is connected to the multiple-tap secondary of a transformer 64 by means of the relay-operated switch 66. The primary of the transformer 64 is connected to suitable A.-C. voltage source. By selecting one of two taps on the secondary of the transformer 64, the relay switch 66 causes the motor 62 to operate at a selected one of two possible speeds.

The output on the amplifier 20 is connected across a voltage divider 68 which has a plurality of taps, as indicated at 70, 72, 74 and 76. Each of these taps may be selectively connected by relay switches, such as indicated at 78, 80, 82 and 84 to one input channel of a two-channel pen recorder 86'. The recorder 86 may be of any suitable type for making a permanent record of the variations in the output of the amplifier 20.

Programming of the operation of the mass spectrometer to effect full automatic control is accomplished through operation of the various relay switches in the circuit thus far described by a punch card control device indicated generally at 88. This device includes a circular disc 90 that may be readily removed and replaced with other similar discs. For example, the disc may be made of heavy chart paper with a metal foil or a conductive paint forming at least one conductive surface. The disc is rotated by the motor drive 62 by any suitable drive means which is preferably arranged to permit the disc 90 to be removed and replaced. A potential is applied to the conductive surface of the disc by means of a potential source, such as a battery 92 connected to a wiper contact spring 94 insulatedly supported from a frame 96. Each of the relay coils of the relay switches 48, 66, and 78, 80, 82 and 84 is connected to its own wiper contact spring, such as indicated respectively at 98, 100, 102, 104, 106 and 108. Thus, a circuit is normally provided through the conductive surface of the disc 90 for energizing the respective relays from the potential source 92.

An additional spring wiper contact 110, insulatedly supported by the frame 96, is connected to the second input channel of the pen recorder 86. The purpose of this wiper contact is to provide mass number marking means on the recorded plot produced by the recorder 86. This is accomplished in a manner which will hereinafter become apparent.

Control of the several relays is. accomplishedby providing appropriately spaced punches in the disc whereby the circuit between the respective wiper contact springs 98110 may be interrupted.

Referring to Fig. 2, an enlarged fragment of the disc 90 is shown with a typical punched hole pattern thereon. By properly tapering the potentiometers 44 and 46 for controlling the specific mass number of the spectrometer in response to the motor drive 62, a substantially linear relationship between the angular position of the drive shaft to the controlling potentiometers 44 and 46 and the resulting specific mass number of the beam striking the collector 18 in the mass spectrometer may be provided. Since the card 90 is driven from the same motor drive means 62 as the potentiometers 44 and 46, it follows that the disc 90 can be divided up into a number of radial positions or segments spaced at equal angles around the disc, each radial position corresponding to a particular specific mass number as established in the mass spectrometer.

In Fig. 2 the different radial positions are identified by their corresponding specific mass number, as indicated at 2, 12, 13, 14, 15, 16 etc. Punches are provided at each of these radial positions at a selected radial distance on the disc corresponding to the radial distance of the wiper contact 110. Thus, as the disc 90 rotates, the circuit through the wiper contact is interrupted with the scanning of each successive specific mass num her. As a result a pip is produced on the graphical record of the recorder 86 by means of which the successive specific mass numbers scanned can be identified.

At radial distances corresponding to the distances of the contacts 102, 104, 106 and 108, punches are provided at each of the radial positions 011 the disc 90 for establishing a predetermined amount of attenuation between the amplifier 20 and the input to the recorder 86 via the voltage divider 68. The disc provides a permanent record of the amount of attenuation introduced for each of the specific mass numbers in interpreting the amplitude of the peaks recorded for the several specific mass numbers.

A punch is provided in the specific mass number 2 radial position at a radial distance on the card corresponding to the radial distance of the contact spring 98 whereby the relay 48 is opened and a lower voltage from the voltage divider 42 used to set the corresponding specific mass number on the mass spectrometer. By this means the mass spectrometer may be caused to skip from a specific mass number 2 to a specific mass number 12 at the same rotation of the disc 90 and potentiometers 44 and 46 as normally would efiect only a change of l in the specific mass number. Thus, the specific mass numbers 3 through 11 can be skipped, which is desirable since these specific mass numbers rarely occur for ionized gases.

If it is further desired to skip rapidly over a number of specific mass numbers within the spectrum, this may be accomplished by providing a continuous slot on the punch card through an angle encompassing the undesired specific mass number segments. The slot is positioned at a radial distance on the disc 90 corresponding to the radial distance of the wiper contact 100. This breaks the circuit to the relay switch 66 causing the motor drive 62 to be connected across the full secondary winding of the transformer 64, resulting in an increased scanning speed by the motor drive 62. At the same time a similar slot can be provided in the disc 90 at a radial distance corresponding to the wiper contact 102 whereby the relay-operated switch 84 is closed, shorting out the input to the recorder 86. Thus, during the rapid scan of the mass spectrometer, no trace is provided on the recorder 86 in response to the output of the amplifier 20.

From the above description it will be seen that an improved automatic mass spectrometer apparatus is provided which achieves a more rapid scan with increased aecuracyof results thancould heretofore beachieved by manually-operated mass spectrometers. The increased scanning rate can be achieved by automatically controlling the attenuation introduced for each of the specific mass numbers, and greater accuracy can be achieved because the gas pressure of the sample can be maintained more constant over the resulting shorter interval of time during which the measurement is made. Furthermore, the chance for human error in noting or recalling the amount of attenuation provided between the amplifier 20 and recorder 86 is obviated, since a permanent record of the amount of attenuation for each specific mass number is provided by the disc 90. The removable disc may be readily replaced to permit difierent programming where different known gases constitute the principal sample constituents.

What is claimed is:

1. An automatic mass spectrometer having an accelerator and a collector, said spectrometer including means for continuously varying the accelerator potential on said accelerator, means for driving said potential varying means, a programmed switching circuit including a punched disc driven in synchronism with the potential varying means, means including a variable step attenuator coupled to the collector for producing an output signal, means for indicating the magnitude of the output signal, means for selectively varying the speed of said driving means in response to the programming circuit, means for selectively varying the step attenuator in response to said programming circuit, and means for connecting a fixed predetermined potential to said accelerator in response to said programming circuit.

2. Apparatus as defined in claim 1, wherein said disc includes a plurality of equally spaced punches, and means responsive to said punches for providing indications correlated with the indication of the variations in output magnitude for relating the output magnitude peaks with the specific mass number of the ions causing those peaks.

3. An automatic mass spectrometer having an accelerator and a collector, said spectrometer including means for continuously varying the accelerator potential on said accelerator, means for driving said potential varying means, a programmed switching circuit including a punched disc driven in synchronism with the potential varying means, means including a variable step attenuator coupled to the collector for producing an output signal, means for indicating the magnitude of the output signal, means for selectively varying the step attenuator in response to said programming circuit, and means for connecting a fixed predetermined potential to said accelerator in response to said programming circuit.

4. An automatic mass spectrometer having an accelerator and a collector, said spectrometer including means for continuously varying the accelerator potential on said accelerator, means for driving said potential varying means, a programmed switching circuit including a punched disc driven in synchronism with the potential varying means, means including a variable step attenuator coupled to the collector for producing an output signal, means for indicating the magnitude of the output signal, means for selectively varying the speed of said driving means in response to the programming circuit, and means for selectively varying the step attenuator in response to said programming circuit.

5. An automatic mass spectrometer having an accelerator and a collector, said spectrometer including means for continuously varying the accelerator potential on said accelerator, means for driving said potential varying means, a programmed switching circuit including a punched disc driven in synchronism with the potential varying means, means including a variable step attenuator coupled to the collector for producing an output signal, means for indicating the magnitude of the output signal, and means for selectively varying the step attenuator in response to said programming circuit.

6. An automatic mass spectrometer having an accelerator and a collector, said spectrometer including means for continuously varying the specific mass number of ions received by the collector, means for driving said specific mass number varying means, a programmed switching circuit including a punched disc driven in synchronism with the mass number varying means, means including a variable step attenuator coupled to the collector for producing an output signal, means for selectively varying the speed of said driving means in response to the programming circuit, and means for selectively varying the step attenuator in response to said programming circuit.

7. An automatic mass spectrometer having an accelerator and a collector, said spectrometer including means for varying the specific mass number of ions received by the collector through the range of specific mass numbers, means for driving said specific mass number varying means, a programmed switching circuit driven by said driving means in synchronism with the specific mass number varying means, the programmed switching circuit including a replaceable program medium for changing the switching sequences performed by the switching circuit, means including a variable step attenuator coupled to the collector for producing an output signal, and means for selectively varying the step attenuator in response to said programming circuit for changing the attenuation of the output signal according to a predetermined pattern established by the programmed switching circuit as the spectrometer is varied through the range of specific mass numbers.

References Cited in the file of this patent UNITED STATES PATENTS 2,650,306 Robinson Aug. 25, 1953 

